How far away is the nearest galaxy?

Most of us know space is a big place, but few of us have an idea of just how big it really is. For instance, how feasible would it be for humans to travel right the way across our galaxy, and would we ever be able to reach other galaxies. In fact, what actually is a galaxy?

OK, if you’re here just to find out how far away our nearest galaxy is, well its 25,000 light years away. But if you’d like to know the answers to the questions above, and want to see a picture of the Millenium Falcon, then read on.

A galaxy is basically a cloud of stars in space. Hundreds of billions of stars, our Sun is one of these stars in our galaxy. Many stars probably have their own planets, so our Solar System is likely one amongst billions of other solar systems in our galaxy. A galaxy is a gravitationally bound system, which means all the stuff in the galaxy is attracted to each other by gravity, and most galaxies rotate, spinning around a central point called the centre of mass, kind of like a spinning plate on a stick. In many galaxies this centre of mass is thought to be a super massive black hole, which is cool.

Galaxies contain other stuff besides stars, planets and black holes, like clouds of gas and dust, asteroids, comets and something ominously called dark matter that scientists don’t yet understand. Galaxies come in a range of types, differing both in size, from dwarf to giant galaxies, and also in shape, with spiral and elliptical galaxies two of the most common. Between galaxies there isn’t really that much, a few molecules of hydrogen and helium gas here and there, and maybe a few specks of rock, but basically, nothing. Our Galaxy is called the Milky Way, it’s a spiral galaxy, and here’s a picture, with our Solar System marked by the white cross.

Until 1924 almost all scientists thought that the Milky Way was the entire Universe; that our galaxy was all there was. I say almost all scientists, as there was an exception. In 1922 – 1923 an astronomer called Edwin Hubble had discovered some distant stars of a type called Cepheid variables, which appeared to be a long way away. Cepheid variables pulse, they intermittently brighten then dim, and astronomers worked out that the more luminous the star the longer the pulse. If you know how luminous the star is you can work out how far away it is, as the brighter the star is in the night sky the closer it is to Earth. Hubble realised that the stars he’d found were so dim that they were extremely far away and that they must be in more distant galaxies. This must have been a mind-blowing moment, when Hubble realised there were other galaxies, and that the Universe was a much larger place than we’d previously thought, I’d love to have seen the look on his face. Today we believe that there are more than 170 billion galaxies in the observable Universe (the Universe is so big, that light can only reach us from a certain distance, so we can’t see beyond this distance, yet).

In most science fiction books and films, even the most advanced star-faring civilizations are constrained to our Galaxy; in Star Trek the Federation has only explored a small part of the Milky Way, and even the Millennium Falcon never left its galaxy, although it dicked all over the castle run. But are sci-fi authors being unnecessarily timid in their imaginings?

Probably not. The Milky Way is roughly 100,000 light years across. A light year is the distance that a beam of light can travel in one year in space. As far as physicists know, light is the fastest moving thing in the Universe, one light year is roughly equal to ten trillion kilometers. This means that the Milky Way is roughly one thousand million billion km across. That’d be a long way then. If a space ship could travel at 50% the speed of light (we may NEVER travel this fast, who knows?) then it would take 200,000 years to travel end-to-end. This is too long a trip for one spaceship, but maybe humans could travel from star to star, creating colonies as they go and eventually cover the entire Milky Way across a number of generations, it took us 200,000 years to create the iPod, maybe in another 200,000 years we’ll be busy conquering our Galaxy.

But what about other galaxies? Our nearest galactic neighbour is called the Canis Major Dwarf Galaxy, its smaller than the Milky Way, with roughly 1 billion stars, and is an irregular elliptical shaped galaxy. It’s about 25,000 light years away from Earth. Compared to the size of out galaxy, the distance isn’t that far, its 12.5% the length of the Milky Way. There is a problem though. To conquer the Milky Way humans could travel from star to star, founding colonies and gathering resources to enable expansion to the next star. To reach the Canis Major Dwarf Galaxy there would be nowhere to stop on the way. If a spaceship travelled at half the speed of light, the journey would take 50,000 years, imagine a community of humans surviving one a spaceship for that long. Hard isn’t it? Stephen Baxter wrote a short story about a very similar scenario called Mayflower II, I don’t want to spoil it for you, but the humans don’t ever get there, and you’d be hard pushed to describe the things living in the ship after thousands of years as human (try and find the story, its superb).

So there you have it, our nearest galaxy is 25,000 light years away (or 250 million billion kilometers), probably too far for humans to ever reach, unless we can travel faster than light. And Einstein says that isn’t possible. Let’s hope he was wrong.

Well written article, and it explains things I didn’t know. One question occurred to me – you said the black holes were thought to be at the center of the galaxies, that the stars rotated around these, and it was cool.
My question is how do astronomers measure the temperature of black holes to determine their temperature.

To be honest, I don’t really know. Black holes do emit a form of radiation called Hawking radiation, so I guess you could derive a temperature for the black hole by measuring this radiation. I imagine it would be extremely cold/low energy though, so I’m not sure if telescopes today are sensitive enough to detect Hawking radiation.

I believe the hypothesis that super massive black holes lie at the heart of some galaxies is based on theoretical modelling of the structure and the rotation of galaxies though, rather than direct observation of the black holes. Basically, many galaxies behave in such a way that suggests they have a region of massive gravitational attraction packed into a relatively small area at their centers. This must be caused by an object with a gigantic mass, and the only object in space that we know off that could pack so much mass into such a small area is a black hole.